It is well known to employ lighting circuits in which an incandescent lamp is normally connected to a line so that it illuminates immediately when the line is supplied with power. A mercury lamp is also connected to the line. It is initially in a cold start condition and requires time to achieve full illumination, whereupon the voltage impressed upon it reaches approximately 135. volts. This increasing voltage is sensed and effects opening of the normally closed circuit of the incandescent lamp. Whenever the mercury lamp is de-energized, as by momentary power failure, its re-firing, now under hot start condition, requires about 5 minutes. During this period, the incandescent lamp is turned on. A system of this type is disclosed in the patents to Haymaker, No. 3,599,036 and Willis, Jr., No. 3,737,720.
Presently employed high pressure mercury vapor lamps of the type frequently employed for street lamps, in gymnasiums, and in stadiums, usually do not reach peak illumination levels until activated for several minutes by an a.c. source. These lamps are also characterized by a lack of illumination for several minutes in response to even a momentary interruption in the a.c. voltage applied to the mercury vapor lamp, as can occur in response to a momentary power shortage or in response to accidental open circuiting of a switch between the power source and mercury vapor lamp. The former and latter low illumination periods are frequently referred to in the art as cold start and hot start conditions, respectively.
There have been numerous systems developed in the prior art to energize an auxiliary lamp, such as an incandescent lamp, during the cold and hot start periods. Some of these prior art systems have included timing circuits for controlling the energization of the auxiliary lamp, while others have included photocells responsive to the illumination level of the mercury vapor lamp.
Some systems include circuitry for detecting the voltage levels across the mercury vapor lamp and a capacitor in a ballast circuit for driving the mercury vapor lamp. During cold start and prior to full illumination of the mercury vapor lamp, the voltage drop across the mercury vapor lamp is below a predetermined level, while the voltage across the capacitor is less than a predetermined level during hot start conditions.
For one particular frequently utilized mercury vapor lamp powered by a 120. volt a.c. source through a typical circuit including a saturable reactor and ballast capacitor, the voltage applied to the mercury vapor lamp is approximately 15. volts immediately after the application of power to the saturable reactor during cold start conditions and, thereafter, builds up gradually over a time period of several minutes, approximately 5 minutes, to approximately 135. volts. During the entire cold start condition and while the mercury vapor lamp is illuminated during normal operation, the a.c. voltage across the ballast capacitor is approximately 420. volts. During hot start conditions, there is initially a very low voltage, on the order of 10. volts, across the ballast capacitor, while a voltage of approximately 270. volts initially exists across the mercury vapor lamp. After several minutes have elapsed the mercury vapor lamp again restrikes, and illumination therefrom begins to increase. Full illumination of the mercury vapor lamp, however, is not achieved until several additional minutes have elapsed. At full illumination 420. volts is developed across the capacitor electrodes and approximately 135. volts is developed across the mercury vapor lamp.